Liquid crystal displays have come to be in use in equipment including household electric appliances, measuring instruments, automotive panels, word processors, electronic organizers, printers, computers, and televisions as well as in clocks and calculators. Representative examples of types of liquid crystal display technologies include TN (twisted nematic), STN (super-twisted nematic), DS (dynamic scattering), GH (guest-host), IPS (in-plane switching), OCB (optically compensated bend), OCB (optically compensated birefringence), ECB (electrically controlled birefringence), VA (vertical alignment), CSH (color super-homeotropic), and FLC (ferroelectric liquid crystal). The current standard driving technique is multiplex driving, replacing the conventional static driving, and the mainstream is the simple matrix technology, or more recently the active matrix (AM) technology, which uses devices such as TFTs (thin-film transistors) and TFDs (thin-film diodes) to drive the display.
A commonly used method for the production of a liquid crystal display is a one-drop filling process in which a photocurable and thermosetting sealant is used. In this method, a rectangular seal pattern is first formed on one of two transparent substrates with electrodes using a dispenser or by screen-printing. Liquid crystal droplets are then applied to the entire inside of the frame on the transparent substrate with the sealant uncured, and the other transparent substrate is immediately bonded to the first one. The seal section is irradiated with ultraviolet radiation for precuring. The sealant is then heated during the annealing of liquid crystals for postcuring to complete the liquid crystal display. Bonding the substrates together under reduced pressure conditions leads to extremely efficient production of liquid crystal displays.
When a photocurable and thermosetting sealant is used for a small liquid crystal display panel, however, complicated metallic wiring or the black matrix overlaps with the pattern of the sealant, shielding the seal pattern from the light for precuring in some areas. In such a shielded area, the uncured sealant can elute into liquid crystals and contaminate them during the process from light irradiation to thermal curing. Liquid crystal panels in recent years tend to use liquid crystals with a low drive voltage requirement (low-voltage liquid crystals) because of reduced power consumption for applications such as mobile use. Owing to their especially high dielectric anisotropy, low-voltage liquid crystals are likely to entrap impurities such as residues in the sealant, e.g., an unreacted polymerization initiator and a cured initiator, chlorine and other ionic impurities, and a silane coupling agent, and this causes the problems of misalignment and a decrease in voltage holding ratio over time.
As a solution to this, a thermosetting sealant for one-drop filling has been proposed that requires no precuring through irradiation with light. Known thermosetting sealants, however, are made from resins that become less viscous when heated and thus still pose some problems such as partial deformation of the seal pattern and damage to the electrical characteristics of the liquid crystal display associated with the elution of ingredients of the sealant into liquid crystals.
Under these circumstances, a proposal has been made to use an epoxy resin with an increased softening point in a sealant to limit the elution of ingredients of the sealant into the liquid crystal material in order to avoid contamination of the liquid crystal material through contact between the liquid crystal material and uncured sealant and reduce color irregularities (PTL 1).
In general, epoxy resins feature high adhesive strength but on the other hand are very liable to contaminate liquid crystal materials. A possible way of reducing the contamination of a liquid crystal material is acrylic modification, and this is expected to reduce the contamination of the liquid crystal material while improving adhesive strength. In some cases, however, acrylic modification has affected thermosetting properties and resulted in contamination of the liquid crystal material by eluted ingredients of the sealant. Another proposal has been made as a solution to this, which suggests that a tertiary amine such as imidazole be added for curing the acrylic component and in order for the acrylic resin to be thermally cured through the interaction between the amine and a small amount of epoxy resin that coexists with the amine (PTL 2).
Furthermore, known thermosetting curable sealants, made from resins that become less viscous when heated, still pose some problems such as partial deformation of the seal pattern and leakage of liquid crystals breaking through the seal pattern. As a solution to this, a composition with improved curability and preserved adhesion to substrates has been proposed (PTL 3).
All of these proposals, however, assume commonly used liquid crystal materials and focus only on sealant composition. These are attempts to avoid issues by finding a new sealant composition and do not necessarily provide satisfactory display characteristics when applied to specific liquid crystal displays. In particular, the phenomenon of image-sticking to a liquid crystal display has not been sufficiently improved.